Resistance spot welding, which is a type of lap resistance welding, is typically used to join overlapping steel sheets.
This welding method is a method to join two or more overlapping steel sheets by applying a high welding current for a short time between a pair of electrodes squeezing the steel sheets from above and below. A point-like weld is obtained using the resistance heat generated by passing the high-current welding current. Such a point-like weld is referred to as a nugget and is the portion where both of the overlapping steel sheets fuse and coagulate at a location of contact between the steel sheets when current is applied to the steel sheets. The steel sheets are joined in a point-like manner by this nugget.
In order to obtain good weld quality, it is important to form the nugget which has an appropriate diameter. The nugget diameter is determined by welding conditions such as the welding current, welding time, electrode shape, electrode force, and the like. Therefore, to form an appropriate nugget diameter, the above welding conditions need to be set appropriately in accordance with the conditions of materials to be welded, such as the material properties, sheet thickness, number of sheets overlapped, and the like.
For example, when manufacturing automobiles, spot welding is performed at several thousand points per automobile, and workpieces that arrive one after another need to be welded. At this time, if the conditions of materials to be welded, such as the material properties, sheet thickness, number of sheets overlapped, and the like are identical, then the welding conditions such as the welding current, welding time, electrode shape, electrode force, and the like are also identical. During consecutive welding, however, the surfaces of the electrodes in contact with the materials to be welded gradually wear, so that the contact area gradually expands beyond that of the initial state. If the same welding current as in the initial state is applied after the contact area has thus expanded, the current density in the materials to be welded lowers, and the temperature rise in the weld is reduced. The nugget diameter therefore decreases. Hence, for every several hundred to several thousand spots of welding, the electrodes are either dressed or replaced, so that the electrode tip diameter does not expand excessively.
A resistance welding device provided with a function (stepper function) to increase the welding current after welding a predetermined number of times, so as to compensate for the reduction in current density due to wear of the electrodes, has also been used conventionally. To use that stepper function, the above-described pattern for changing the welding current needs to be set appropriately in advance. Performing tests or the like, however, to derive a pattern for changing the welding current that corresponds to numerous welding conditions and conditions of materials to be welded is highly time-consuming and expensive.
The state of progress of electrode wear also varies during actual work. Therefore, the predetermined pattern for changing the welding current cannot always be considered appropriate.
Furthermore, when there is a disturbance at the time of welding, such as when a point that has already been welded (previously welded point) is located near the point being welded, or when the surface of the materials to be welded is highly uneven and a contact point between the materials to be welded is located near the point being welded, then current diverts to the previously welded point or the contact point. In such a state, the current density is reduced at the position to be welded directly below the electrodes, even when welding under predetermined conditions. A nugget of sufficient diameter therefore cannot be obtained.
In order to compensate for this insufficient amount of heat generated and to obtain a nugget of sufficient diameter, it becomes necessary to set a high welding current in advance.
Techniques such as the following have been proposed to resolve the above problem.
For example, JP H9-216071 A (PTL 1) discloses a control unit of a resistance welder that obtains a set nugget by comparing an estimated temperature distribution of the weld with a target nugget and controlling output of the welder.
JP H10-94883 A (PTL 2) discloses a method of controlling welding conditions of a resistance welder to achieve good welding by detecting the welding current and the voltage between tips, performing a simulation of the weld by heat transfer calculation, and estimating the formation state of the nugget.
Furthermore, JP H11-33743 A (PTL 3) discloses the achievement of a good weld, regardless of the type of materials being welded or the wear state of the electrodes, with a welding system that first uses the sheet thickness of the materials being welded and the welding time to calculate the cumulative amount of heat generated per unit volume that allows for good welding of the materials being welded and then adjusts the welding current or voltage that yields the calculated amount of heat generated per unit volume and unit time.